Corrected entry: During the attack on the Banking Clan HQ, one of the Clone Army SPHA-Ts fires a shot in a ballistic arc. Beam Cannons do not  and cannot  fire in a ballistic arc; this happens only when a solid projectile or shell is dragged down by gravity as it flies

Correction:The projectile arcs because it has been modified. The weaponry we see in the movie is an SPHA-M. It modifies the projectile before it leaves the barrel. This is somthing you would need to look into.